It's a snippet of HTML code that, when executed by a user’s browser, makes a call to Ad Manager.
Advertising - Google Publisher Tag
<script async='async' src='https://www.googletagservices.com/tag/js/gpt.js'></script>
<script>
var googletag = googletag || {};
googletag.cmd = googletag.cmd || [];
googletag.cmd.push(function() {
googletag.defineSlot(
'/43392404/Ad_Manager_Entertainment', // the network id and the ad unit path
[[728, 90], [160, 600], [300, 250]], // the ad size
'div-gpt-ad-1553186525518-0' // the id of the div element (the slot) that will receive the ad creative (It can anything as long as it’s unique to the page)
).addService(googletag.pubads());
googletag.pubads().enableSingleRequest(); // Single Request Mode means that instead of sending individual ad requests to Ad Manager servers, the browser is able to send one request notifying the server of all ad units on the page. This enables guaranteed roadblocking and improves page load time.
googletag.pubads().setTargeting('age', ['18 – 24']); // Key-values
googletag.enableServices();
});
</script>
where:
AdManager can create it for you. From the Inventory section, click Ad Units and select the ad unit for which you’d like to generate the tag. Click the Generate Tags button.
More … Doc - Generate ad tag